1. Field of the Disclosure
This disclosure relates to turbochargers with variable turbine geometry with an actuator pivot shaft. Specifically, this disclosure relates to replacing a standard actuator shaft bushing with a rolling-element bearing.
2. Description of Related Art
Advantages of turbocharging include increased power output, lower fuel consumption and reduced pollutant emissions. The turbocharging of engines is no longer primarily seen from a high-power performance perspective, but is rather viewed as a means of reducing fuel consumption and environmental pollution on account of lower carbon dioxide (CO2) emissions. Currently, a primary reason for turbocharging is using exhaust gas energy to reduce fuel consumption and emissions. In turbocharged engines, combustion air is pre-compressed before being supplied to the engine. The engine aspirates the same volume of air-fuel mixture as a naturally aspirated engine, but due to the higher pressure, thus higher density, more air and fuel mass is supplied into a combustion chamber in a controlled manner. Consequently, more fuel can be burned, so that the engine's power output increases relative to the speed and swept volume.
In exhaust gas turbocharging, some of the exhaust gas energy, which would normally be wasted, is used to drive a turbine. The turbine includes a turbine wheel that is mounted on a shaft and is rotatably driven by exhaust gas flow. The turbocharger returns some of this normally wasted exhaust gas energy back into the engine, contributing to the engine's efficiency and saving fuel. A compressor, which is driven by the turbine, draws in filtered ambient air, compresses it, and then supplies it to the engine. The compressor includes a compressor impeller that is mounted on the same shaft so that rotation of the turbine wheel causes rotation of the compressor impeller.
Turbochargers typically include a turbine housing connected to the engine's exhaust manifold, a compressor housing connected to the engine's intake manifold, and a center bearing housing coupling the turbine and compressor housings together. The turbine housing defines a volute that surrounds the turbine wheel and that receives exhaust gas from the engine. The turbine wheel in the turbine housing is rotatably driven by an inflow of exhaust gas supplied from the exhaust manifold.
A variable turbine geometry (VTG) turbocharger allows a turbine flow cross-section leading to the turbine wheel to be varied in accordance with engine operating points. This allows the entire exhaust gas energy to be utilized and the turbine flow cross-section to be set optimally for each operating point. As a result, the efficiency of the turbocharger and hence that of the engine can be higher than that achieved with bypass control of a wastegate valve assembly. Variable guide vanes in the turbine have an effect on pressure build-up behavior and, therefore, on the turbocharger power output.
A VTG pivot shaft is typically not fitted directly to a bore in the turbine housing, but more often to a stationary bearing in a bore in the turbine housing. The pivot shaft is often radially located in a bearing, which can be located either in a bore, with a centerline within the turbine housing, or directly in the bearing housing depending on the design.
Borg Warner's US 2011/037639 for a “Control Shaft Seal” discloses a method for keeping an “actuator pivot shaft” concentric in its bore to enhance the sealing capability thus maximizing the retention of the exhaust gases within the turbocharger and minimizing the escape of exhaust gases to the environment external to the turbocharger.
Other VTG actuator pivot shafts can experience significant wear in certain applications during operation, presumably from high pressure loading between the bushing and shaft near the fork end of the shaft (turbine side). Lack of lubrication can cause high friction forces between the shaft and bearing components, reducing available VTG actuation torque. Furthermore, in certain operations, it may be difficult to seal the shaft to the bearing housing because of clearance between the shaft and bushing and the associated misalignment of the shaft to the bushing/housing.
This disclosure focuses on a rolling-element bearing for a VTG actuator pivot shaft, and forces and movement between the shaft and bearing components.